Lubricant compositions

ABSTRACT

Lubricant films are formed on solid substrates by application of a composition containing solid lubricant particles carried in a dispersion of colloidal silica in an acidified alcohol-water solution of the partial condensate of methyltrisilanol.

United States Patent 1191 Clark 51 Sept. 30, 1975 LUBRICANT COMPOSITIONS [75] Inventor: Harold A. Clark, Midland, Mich.

[73] Assignee: Dow Corning Corporation, Midland,

Mich.

22 Filed: June 24,1974

211 Appl. No.: 482,297

[52] US. Cl. 252/12 [51] Int. Cl. ClOM 5/00; ClOM 7/00 [58] Field of Search 252/12, 12.2, 12.4, 12.6

[56] References Cited UNITED STATES PATENTS 3,467,596 9/1969 Hermann 252/ l 2 3,516,933 6/1970 Andrews et al i 252/12 Primary ExaminerDelbert E. Gantz Assistant E.\'aminerl. Vaughn Attorney, Agent, or FirmNorman E. Lewis 5 7 ABSTRACT 10 Claims, No Drawings LUBRICANT COMPOSITIONS BACKGROUND OF THE INVENTION The present invention relates to solid lubricant compositions. In one aspect the invention relates to substrates coated with a film of dry solid lubricant. In another aspect, the invention relates to a composition containing a novel film forming carrier for particulate solid lubricants.

Dry film lubricants are used to provide solid surfaces with a dry coating having some degree of permanance which serves as a lubricant in that it reduces the coefficient of friction. This class of lubricants usually consists of particles of a solid lubricant dispersed in film-forming resin matrix.

A number of different resins have been proposed for use as the binder component of the lubricant film. The most commonly used resin in dry film lubricants are the phenolic resins, including modified phenolics, epoxy resins and acrylic polymers. Silicones have been used to improve the heat-stability and load-bearing capacity of lubricant films see, for example, the disclosure of U.S. Pat. No. 3,288,710. Air drying solvent soluble silicones are proposed as the sole binder for dry lubricants in U.S. Pat. No. 3,674,690.

The present invention provides lubricant compositions suitable for use in forming dry films which'represent an improvement over the prior art in that the binder material is heat-stable, abrasion and corrosion resistant and is relatively inexpensive.

Thus, it is an object of the present invention to provide a novel lubricant composition. It is another object of the invention to provide substrates coated with an effective dry film lubricant. A further object of the present invention is to provide an improved filmforming carrier system for dry lubricants. These and other objects of the invention will be apparent to one skilled in the art upon consideration of the following detailed description of the invention and appended claims.

DETAILED DESCRIPTION OF THE INVENTION In accordance with the present invention, there is provided a lubricant composition comprising a homogeneous mixture of (a) a particulate solid lubricant and (b) a film-forming carrier consisting essentially ofa dispersion of colloidal silica in a lower aliphatic alcoholwater solution of the partial condensate of a silanol of the formula RSi(OH) in which R is selected from the group consisting of alkyl radicals of 1 to 3 inclusive carbon atoms, the vinyl radical, the 3,3,3-trifluoropropyl radical, the gamma-glycidoxypropyl radical and the gamma-methacryloxypropyl radical, at least 70 weight percent of the silanol being CH;,Si(OI-I) said carrier containing to 50 weight percent solids consisting essentially of 10 to'75 weight percent colloidal silica and 25 to 90 weight percent of the partial condensate, said carrier containing sufficient acid to provide a pH of 3.0 to 6.0 in the carrier.

Component (a) of the lubricant composition can be any finely divided particulate solid lubricant. Suitable solid lubricants include molybdenum disulfide, molybdenum diselenide, graphite, antimony trioxide, titanium disulfide, tungsten disulfide, tungsten diselenide, bismuth trioxide, lead sulfide, lead oxide, magnesium oxide, boron nitride, calcium fluoride, zirconium disulfide, colloidal silver and the like. Other suitable solid lubricants include finely divided polymeric materials having inherent lubricity, subh as polytetrafluoroethylene (powdered Teflon), polytrifluoromonochloroethylene (Kel-F), nylon, polyalkylenes, such as polypropylene and copolymers thereof, polyimides and aromatic polyesters, such as poly(p-oxybenzoyl). In addition to providing lubricity, certain of the polymers, such as polytetrafluoroethylene and poly(poxybenzoyl), impart non-stick properties to the film. Of course, combinations of solid lubricants can be used, such as mixtures of graphite and molybdenum disulfide, mixtures of calcium fluoride and boron nitride and mixtures of antimony trioxide and molybdenum disulfide. These solid lubricants are available as finely divided powders generally having a particle size in the range of l to micron diameter. A further discussion of solid lubricants can be found in Braithwaites Solid Lubricants and Surfaces, The Macmillan Company, New York (l964).

As described above, the film forming carrier for the solid lubricant is a dispersion of colloidal silica in an alcohol-water solution of the partial condensate of a trisilanol. When cured the carrier forms a binder film of silica and silsesquioxane. The major portion of the partial condensate or siloxanol is Obtained from the condensation of CH Si(OI-I) a minor portion, if desired, being obtained from cocondensation with 2 5 )=i :x 1 :i):h F .-;)3,

and

or mixtures thereof. From both the standpoint of economy and optimum properties in the cured film it is preferred to utilize all monomethyltrisilanol in formualtin g the composition.

As will be described in detail in the examples, the trisilanols are generated in situ by adding the corresponding trialkoxysilanes to acidic aqueous dispersions of colloidal silica. Suitable trialkoxysilanes are those containing methoxy, ethoxy, isopropoxy and t-butoxy substituents, which upon hydrolysis liberate the corresponding alcohol; thus, generating at least a portion of the alcohol present in the carrier composition. Upon generation of the silanol in the acidic aqueous medium, there is condensation of the hydroxyl substituents to form Si-O-Si bonding. The condensation is not complete, but rather the siloxane retains an appreciable quantity of silicon-bonded hydroxyl groups, thus rendering the polymer soluble in the water-alcohol sol-. vent. This soluble partial condensate can be characterized as a siloxanol polymer having at least one siliconbonded hydroxyl group per every three SiO units. During curing of the film on a substrate, these residual hydroxyls condense to give a silsesquioxane, RSiO The silica component of the carrier composition is present as colloidal silica. Aqueous colloidal silica dispersions generally have a particle size in the range of 5 to I50 millimicrons in diameter. These silica hydrosols are prepared by methods well-known in the art and are commercially available under such registered trademarks as Ludox and Nalcoag. It is preferred to use colloidal silica of -60 millimicron particle size in order to obtain dispersions having a greater stability and to provide films having superior properties. Colloidal silicas of this type are relatively free of Na O and other alkali metal oxides, generally containing less than 2 weight percent, preferably less than 1 weight percent Na O. They are available as both acidic and basic hy drosols. Colloidal silica is distinguished from other water dispersable forms of SiO-;, such as polysilicic acid or alkali metal silicates, which are not operative in the practice of the present invention.

The silica is dispersed in a solution of the siloxanol carried in alcohol-water cosolvent. Suitable lower aliphatic alcohols include methanol, ethanol, isopropanol, and t-butyl alcohol. Mixtures of such alcohols can be used. lsopropanol is the preferred alcohol and when mixtures of alcohol are utilized it is preferred to utilize at least 50 weight percent of isopropanol in the mixture to obtain optimum adhesion of the coating. The solvent system should contain from about to 75 weight percent alcohol to ensure solubility of the siloxanol. Optionally, one can utilize an additional water-miscible polar solvent, such as acetone, butyl cellosolve and the like in a minor amount, for example, no more than 20 weight percent of the cosolvent system.

To obtain optimum properties in the film and to maximize bath life of the coating composition, sufficient water-miscible organic acid to provide a pH of from 3.0 to 6.0 must be present. Suitable acids include both organic and inorganic acids such as hydrochloric, acetic, chloroaeetic, citric, benzoic, dimethylmalonic, formic, glutaric, glycolic, maleic, malonic, toluene-sulfonic, oxalic and the like. The specific acid utilized has a direct effect on the rate of silanol condensation which in turn determines shelf life of the composition. The stronger acids, such as hydrochloric and toluenesulfonic acid, give appreciably shortened shelf or bath life and require less ageing to obtain the described soluble partial condensate. It is preferred to add sufficient water-miscible carboxylic acid selected from the group consisting of acetic, formic, propionic and maleic acids to provide pH in the range of 4 to 5.5 in the binder composition. In addition to providing good bath life, the alkali metal salts of these acids are soluble, thus allowing the use of these acids with silicas containing a substantial (greater than 0.2% Na O) amount of alkali metal or metal oxide.

The carrier compositions are easily prepared by adding trialkoxysilanes, such as RSi(OCH to colloidal silica dispersion and adjusting the pH to the desired level by addition of the organic acid. The acid can be added to either the silane or the dispersion prior to mixing the two components, provided that the mixing is done rapidly. The amount of acid necessary to obtain the desired pH will depend on the alkali metal content of the silica but is usually less than one weight percent of the composition. Alcohol is generated by hydrolysis of the alkoxy substituents of the silane, for example, hydrolysis of one mole of -Si(OC H,=,) generated 3 moles of ethanol. Depending upon the percent solids desired in the final composition, additional alcohol, water or a water-miscible solvent can be added. The composition should be well mixed and allowed to age for a short period of time to ensure formation of the partial condensate. The carrier composition thus obtained is a clear or slightly hazy low viscosity fluid.

which is stable for several days. The condensation of SiOH continues at a very slow rate and the composition will eventually form gel structures. The bath life of the composition can be extended by maintaining the dispersion at below room temperature, for example at 40F.

Buffered latent condensation catalysts can be added to the carrier so that milder curing conditions can be utilized to obtain the optimum film properties in the final coating. Alkali metal salts of carboxylic acids, such as potassium formate, are one class of such latent catalysts. The amine carboxylates and quaternary ammonium carboxylates are other classes of latent catalysts. Of course the catalysts must be soluble or at least miscible in the cosolvent system. The catalysts are latent to the extent that at room temperature they do not appreciably shorten the bath life of the composition, but upon heating the catalyst dissociates and generates a catalytic species active to promote condensation, for example an amine. Buffered catalysts are used to avoid effects on the pH of the composition. Certain of the commercially available colloidal silica dispersions contain free alkali metal which reacts with the organic acid during the adjustment of pH to generate the carboxylate catalyst in situ. This is particularly true when starting with a hydrosol having a pH of 8 or 9. The acidic binder can be catalyzed by addition of carboxylates such as dimethylamine acetate, ethanolamine acetate, dimethylaniline formate, tetraethylammonium benzoate, sodium acetate, sodium propionate, sodium formate or benzyltrimethylammonium acetate. The amount of catalyst can be varied depending upon the desired curing condition, but at about 1.5 weight percent catalyst in the composition, the bath life is shortened and physical properties of the film may be impaired. It is preferred to utilize from about 0.05 to 1 weight percent of the catalyst.

To provide the greatest stability in the dispersion form while obtaining optimum properties in the cured coating, it is preferred to utilize a coating composition having a pH in the range of 4-5 which contains l025 weight percent solids; the silica portion having a particle size in the range of 560 millimicrons; the partial condensate of CH ,Si(OH being present in an amount in the range of 35 to 55 weight percent of the total solids in a cosolvent of methanol, isopropanol and water, the alcohols representing from 30 to 60 weight percent of the cosolvent and a catalyst selected from the group consisting of sodium acetate and benzyltrimethylammonium acetate being present in an amount in the range of 0.05 to 0.5 weight percent of the composition. Such a composition is relatively stable, having a bath life of approximately one month, and, when coated on to a substrate, can be cured in a relatively short time at temperatures in the range of l25C.

In addition to component (a), the solid lubricant, and component (b), the carrier, the compositions of the invention can contain minor amounts of conventional additives, such as thiotropic agents, for example, carboxy methyl cellulose, corrosion inhibitors, dyes, oxidation inhibitors and the like.

The lubricant compositions of the invention are formulated by mixing the particulate solid lubricant with the film-forming carrier under high-speed, high-shear conditions. These conditions ensure complete mixing and breaking up of any agglomerations of the solid lu- In addition to application by spraying, the'lubricant compositions can be applied by brushing, dipping or other conventional technique to obtain a coherent uniform film on a solid substrate. The film thickness can be varied with the method of coating but generally film thickness of from 0.1 to 5 mils are utilized.

With proper pretreatment, includingthe use of conventional primers, the coating can be adhered to virtually all types of substrates. Typical pretreatments of metals include phospating of ferrous alloys, oxidation of nickel-containing alloys and anodizing of titanium.

While the coating of metal substrates with the lubricant is of principal interest, the lubricant film can be applied to other solid substrates, such as solid polymeric materials, for example, polyesters, polyamides and the like; cellulosic substrates, especially paper products to impart release properties; ceramics and the like.

Once applied to the substrate surface, the lubricant composition will air dry to give a lubricant film. Generally, room temperature curing is complete in 24 hours. Curing can be accelerated by heating for example at 175F for four hours. The curing mechanism is that of condensation of the residual silanol present in the partial hydrolyzate to provide a binder of RSiO and colloidal silica having particles of the solid lubricant incorporated therein. The largely inorganic nature of RSi- O SiO matrix results in heat stability superior to that obtained using organic resin binders.

From the above discussion it is apparent that the scope of the present invention includes solid substrates having at least one surface thereof coated with a lubricant film comprising particles of a solid lubricant uniformly incorporated in a matrix of RSiO and colloidal silica, the matrix consisting essentially of to 75 weight percent SiO and 25 to 90 weight RSiO In such a cured film the solid lubricant particles are preferably present in an amount in the range of to 60 weight percent based on the total weight of the composition.

The following examples are illustrative and not intended as limiting of the invention delineated in the claims.

EXAMPLE 1 v Sufficient glacial acetic acid was added to grams of an aqueous dispersion containing weight percent of 13-14 millimicron colloidal silica (0.32% Na O and initial pH of 9.8) to reduce the pH to 4.8. Methyltrimethoxysilane (17 grams) was added to the acidified dispersion. This binder composition was aged for two hours with agitation to ensure formation of the partial condensate. The binder composition contained 36 percent solids, half of which were colloidal silica, carried in a methanol-water cosolvent for the partial condensate.

After two hours the binder composition was blended with 25 grams of a commercially-available aqueous emulsion of particulate poly(tetrafluoroethylene) which contained 60-weight percent solids. The emuls'ion was alkaline and acetic acid was added to the blend to adjust the pH to 4.8.

This lubricant composition was coated onto phosphate-treated steel panels. The coating was air-dried for 'one hour and then cured at 135C for 15 minutes. The coated panel was heated on a 300F hot plate and utilized as a cooking surface to fry egg whites. After frying until thoroughly coagulated, the egg white was readily removed with a spatula from the coated surface. Cookies baked for 15 minutes at 425F on the coated 'panel were also readily removed from the surface.

These data demonstrate the use of the lubricant coating as a foodrelease surface.

EXAMPLE 2 The binder composition described in Example 1 was mixed with various solid lubricants to formulate coating compositions which were applied to the surfaces of wear test blocks. The lubricant formulations each contained 50 grams of the 36% solids binder composition, 50 grams of the particulate solid lubricant and 94 grams of isopropanol. When cured, the compositions provided a coating containing 66.6 percent lubricant and 33.3 percent binder, 50 weight percent of the binder being silica, the remainder being CH SiO After curing the coatings on the wear test blocks, lubricant properties were determined by the LFW-l test method. In obtaining the LFW-l results, an Alpha test machine (described in U.S. Pat. No. 3,028,746) was operated in the unidirectional mode using a standard steel ring (R 60) and a R,.3O block at 72 rpm with a load of 210 lbs. The coatings were tested to failure which was defined as the number of revolutions necessary to reach coefficient of friction of 0.166 or more. Duplicate samples were tested. Results are tabulated below.

Solid Lubricant Revolutions These results demonstrate the relative effectiveness of various solid lubricants when utilized in the compositions of the invention.

Reasonable modification and variation are within the scope of the invention which is directed to lubricant compositions and articles coated with such compositions. i

That which is claimed is:

l. A lubricant composition comprising a homogeneous mixture of a. a particulate solid lubricant and b. a film-forming carrier consisting essentially of a dispersion of colloidal silica in a lower aliphatic alcohol-water solution of the partial condensate of a silanol of the formula RSi(OH) in which R is selected from the group consisting of alkyl radicals of l to 3 inclusive carbon atoms, the vinyl radical, the 3 ,3 ,3-trifluoropropyl radical, the gamm aglycidoxypropyl radical and the gamma-methacryloxypropyl radical, at least weight percent of 7 8 the silanol being CH;,Si(Ol-l);;, said carrier contain- 5. A lubricant composition in accordance with claim ing 10 to 50 weight percent solids consisting essen- 4 wherein the catalyst is an alkali metal salt of a carboxtially of 10 to 75 weight percent colloidal silica and li id 25 to 90 weight Pcrccnt of the Paltiall condensate, 6. A lubricant composition in accordance with claim said carried containing sufficient acid to provide a 5 5 wherein the Catalyst is sodium acetate 2 I E f' to m h a d h l 7. A lubricant composition in accordance with claim A compoltlon m 2 wherein the particulate solid lubricnat (a) is present 1 in WhlCh the alcohol is present in an amount m the in an amount in the range of to 75 weight percent range of 20 to 75 weight percent based on the total based on the total weight of composition weight of the cosolvent. 10

3. A lubricant composition in accordance with claim A c9mposn,lon m accPrdance with Claim 2 in which the acid is a carboxylic acid selected from 2 g 'l g the Sohd lubncam (a) polyuctrafluclc' et y ene the group consisting of acetic acid, formic acid, propionic acid and maeic acid 9. A lubricant composition in accordance with claim 4. A lubricant composition in accordance with claim 2 wherein the Solid lubricant is graphite- 3 in which the carrier contains from 0.1 to 1.5 weight A lubricant composition accordance with percent of a buffered latent silanol condensation cata-t claim 2 wherein the solid lubricant (a) is M08 lyst.

UNITED STATES PATENT OFFICE CETIFEQATE 0F ECTEN Patent No. 5,909,4 4 Dated September 5 975 Inventor(s) Harold A. Clark It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Claim 1, column 7, line 5, "said carried containing sufficient" should read said carrier containing sufficient ca ed this fined and A ms t:

RUTH C. MASON C. MARSHALL DANN Attesn'ng Officer Commissioner qflatc'nrs and Trademarks 

1. A LUBRICANT COMPOSITION COMPRISING A HOMOGENEOUS MIXTURE OF A. A PARTICULATE SOLID LUBRICANT AND B. A FILM-FORMING CARRIER CONSISTING ESSENTIALLY OF A DISPERSION OF COLLOIDAL SILICA IN A LOWER ALIPHATIC ALCOHOL-WATER SOLUTION OF THE PRTIAL CONDENSATE OF A SILANOL OF THE FORMULA RSI(OH)3 IN WHICH R IS SELECTED FROM THE GROUP CONSISTING OF ALKYL RADICALS OF 1 TO 3 INCLUSIVE CARBON ATOMS THE VINYL RADICAL THE O83,3-TRIFLUOROPROPYL RADICAL THE GAMMA-GLYCIDOXYPROPYL RADICAL AND THE GAMMAMETHACRYLOXYPROPYL RADICAL AT LEAST 70 WEIGHT PERCENT OF THE SILANOL BEING CH3SI(OH)3 SAID CARRIER CONTAINING 10 TO 50 WEIGHT PERCENT SOLIDS CONSISTING ESSENTIALLY OF 1/ TO 75 WEIGHT PERCENT COLLOIDAL SILICA AND 25 TO 90 WEIGHT PERCENT OF THE PARTIAL CONDENSATE SAID CARRIED CONTAINING SUFFICIENT ACID TO PROVIDE A PH OF 3.0 TO 6.0 IN THE CARRIER.
 2. A lubricant composition in accordance with claim 1 in which the alcohol is present in an amount in the range of 20 to 75 weight percent based on the total weight of the cosolvent.
 3. A lubricant composition in accordance with claim 2 in which the acid is a carboxylic acid selected from the group consisting of acetic acid, formic acid, propionic acid and maleic acid.
 4. A lubricant composition in accordance with claim 3 in which the carrier contains from 0.1 to 1.5 weight percent of a buffered latent silanol condensation catalyst.
 5. A lubricant composition in accordance with claim 4 wherein the catalyst is an alkali metal salt of a carboxylic acid.
 6. A lubricant composition in accordance with claim 5 wherein the catalyst is sodium acetate.
 7. A lubricant composition in accordance with claim 2 wherein the particulate solid lubricnat (a) is present in an amount in the range of 10 to 75 weight percent based on the total weight of composition.
 8. A lubricant composition in accordance with claim 2 wherein the solid lubricant (a) is poly(tetrafluoroethylene).
 9. A lubricant composition in accordance with claim 2 wherein the solid lubricant (a) is graphite.
 10. A lubricant composition in accordance with claim 2 wherein the solid lubricant (a) is MoS2. 